Devices manipulating light signals in wavelength domain are essential to wavelength division multiplexed (WDM) communication systems, which are believed to be a promising way to achieve a few Tb/s of transmission capacity. Much research effort has been directed to optical interferometer filters in order to realize useful WDM devices such as a WDM channel divider, multiplexer, and demultiplexer.
However, since fiber interferometers are usually very sensitive to environmental perturbations such as temperature and vibration, a stabilization scheme is required to make the interferometers practical devices.
FIG. 1 shows a diagram illustrating an optical filtering system with a conventional transmission-type fiber-optic Mach-Zehnder interferometer (MZI) filter.
As shown in FIG. 1, a stabilized MZI filter includes an input terminal that receives input light, optical fiber nodes (optical fiber couplers), output terminals, and optical fibers. Light supplied to the input terminal is divided into two lights at the first optical fiber node. After propagating different optical paths, divided lights are combined and they interfere with each other at the second optical fiber node. If the polarization states of the two Interfered lights are assumed to be identical, the output of the optical filter can be described as follows: ##EQU1##
where n(.lambda.) is the refractive index of the fiber core and .DELTA.l is the fiber length difference between the interferometer's two arms.
As shown in Equation 1, if optical fibers are subject to environmental perturbations such as temperature and vibration, refractive index and the length of the fiber vary and thereby the transmission wavelength of the filter changes. To overcome such difficulties, a stabilized fiber-optic MZI filter system was introduced. This technique has been disclosed in "Environmentally Stable Monolithic Mach-Zehnder Device," U.S. Pat. No. 5,295,205. However, when interferometers are implemented by optical fibers in the fiber-optic MZI filter system, the extinction ratio of the optical filter varies because polarization states of two interfered lights fluctuate slowly and randomly.
FIG. 2 shows a diagram illustrating an optical filtering system with the conventional fiber-optic polarimetric interferometer. As shown therein, it includes an input terminal, a polarizer, a polarization beam splitter (PBS), a first optical fiber node A, a second optical fiber node B, and output terminals.
The polarizer polarizes light supplied to the input terminal. Polarized light propagates through one of two birefringent axes of a polarization maintaining fiber (PMF). At the first optical fiber node A where the first PMF is connected to the second one with the angle of 45 degree between their birefringent axes, the polarized light is divided to the two birefringent axes of the second PMF with the same intensity. After the divided lights propagate through two birefringent axes of the second PMF, they are combined at the second optical fiber node B where the second PMF is connected to the third one with the angle of 45 degree between their birefringent axes.
There are two interference signals in the third PMF since each birefringent axis of the third PMF carries an interference signal. The two interference signals are separated from each other by the polarization beam splitter, and the two divided interference signals are supplied to the first output terminal and the second output terminal, respectively.
In FIG. 2, n.sub.e and n.sub.o represent two birefringent axes of a PMF. Point A and point B in the figure, where two PMF's are connected with 45 degree angle, are counterparts of the two optical fiber couplers of the MZI filter in FIG. 1. The two birefringent axes of the PMF located between the point A and the point B are counterparts of the two optical paths of the MZI filter in FIG. 1. Therefore, if the length of the optical fiber between A and B is represented by l and modal birefringence of the PMF is represented by B, output of the optical filter is described as follows. ##EQU2##
As shown in equation 2, transmission wavelength spacing is controlled by the length of PMF, l.
Extinction ratio variation due to the random fluctuation of the polarization states of two interfering signals can be solved in the PMF interferometer filter system since the PMF preserves the polarization states. In addition, the PMF interferometer filter system is less affected by environmental perturbations than MZI filter system since it has two optical paths in one optical fiber. Utilizing the above two advantages of the PMF interferometer filter system is discussed by K. Okamoto, T. Morioka, I. Yokohama, and J. Noda in an article entitled "All-Panda-Fiber Multi/Demultiplexer Utilizing Polarization Beat Phenomenon In Birefringent Fibers" in IEE Electronics Letters Vol. 22, No. 4, 181-182 (1986).
However, the stability problem resulting from environmental perturbations still remains, which must be solved in order to make the PMF interferometer filter system more practical.